1. Field of Invention
The present invention relates to a rotor yoke and a balance adjusting method thereof.
2. Description of the Related Art
A cooling fan is often used to cool electronic devices and heat-generating electronic parts. In order to improve the cooling performance of the cooling fan, it is necessary to increase the flow rate and static pressure of the cooling fan.
In the cooling fan, an impeller needs to be rotated at a high speed in order to assure the increased flow rate and static pressure. However, vibration is generated as the impeller rotates at high speed. This vibration is caused by the imbalance occurring when a rotor united with the impeller rotates. The imbalance is equivalent to the deviation of the center of gravity of the rotor with respect to a rotational axis. The rotor of the cooling fan includes a rotor yoke, a rotor magnet, and a shaft as well as the impeller.
Particularly, because the rotor yoke is made of a metallic material and has a mass greater than that of the impeller made of a resin, the imbalance of the rotor yoke heavily affects the vibration of the impeller. The vibration present during the high-speed rotation of the impeller deteriorates the characteristics of the cooling fan and reduces the life span thereof. Additionally, the vibration may sometimes be a cause of noise, and noise reduction is strongly desired in household electronic devices. For that reason, a demand has existed for a way to adjust the rotational balance of the rotor, particularly the rotational balance of the rotor yoke.
Japanese Patent Laid-open Publication No. 2002-272069 discloses a disk motor for optical disk devices wherein the eccentricity of a rotor assembly and hence the rotational balance thereof are adjusted by providing a notch in a portion of a rim formed around the outer periphery of a lower opening of a rotor yoke.
With the technique disclosed in Japanese Patent Laid-open Publication No. 2002-272069, the rotor yoke is produced and then the rotational balance thereof is measured. Based on the results of the measurement, a notch is provided in a portion of the rim to adjust the rotational balance. However, in this technique, the rotational balance needs to be adjusted on a yoke-by-yoke basis through post-processing, and this creates the problem of non-productivity.